This invention relates to processes for cold working an elongated form of a steel alloy, and in particular to a process and apparatus for treating the surface of such an article to facilitate cold working thereof.
An elongated form of a steel alloy, such as wire, rod, or bar, can be cold worked to provide useful product sizes and shapes. Here and throughout this application, the term "cold working" means the plastic deformation of metal under conditions of temperature and strain rate that result in strain hardening of the metal and which is typically, but not necessarily conducted at room temperature. Cold working of an elongated form includes such operations as drawing, extruding, cold heading, or a combination thereof. Prior to being cold worked, an elongated form of steel alloy is treated to remove surface conditions such as oxide scale or residue that result from upstream processing, e.g., hot or cold rolling. As a result of such pre-cold-work processing, a smooth surface is formed on the elongated, steel alloy form.
One of the known processes for providing a smooth surface on the elongated steel form is the removal of a thin peripheral layer of material by shaving, peeling, or broaching the elongated, steel alloy form. Here and throughout this application, the term "shaving" includes any operation that is also commonly referred to as "scalping". The shaving, peeling, or broaching process exposes a new surface on the steel alloy form that is very smooth and reflective. Steel wire that has been previously cold-worked can be bright strand annealed prior to further cold working. Such processing also leaves a very smooth and highly reflective surface on the wire.
An elongated steel alloy form having a smooth, reflective surface is difficult to cold work because such a surface is not conducive to carrying lubricant into the cold working tool or die. Consequently, the elongated form of steel alloy is subject to scratching or galling and the cold working tool or die is subject to damage.
Some machines that perform surface layer removal leave superficial, machine tool marks on the surface of the elongated form. These tool marks act as high pressure points that also adversely affect the drawability or extrudability of the steel alloy form. Tool marks that are left on the surface of the elongated form can remain visible after cold working and present an unsightly appearance.
After the surface layer removal process, steel alloy bar, wire, or rod has been treated by acid etching, annealing, and then acid cleaning, or by annealing, shot blasting, and then acid dipping, to provide a dull, etched surface that is more conducive to retaining lubricant during a subsequent cold working operation. However, the use of acids to provide surface etching and scale removal leaves much to be desired for a number of reasons. Acids are highly corrosive and are difficult to dispose of in an environmentally safe manner. Acid cleaning is not readily adaptable to in-line or strand processing. Furthermore, many steel alloys are subject to intergranular attack when the surface is acid cleaned.
The use of shot blasting with metallic shot to treat the surface of an elongated steel alloy form also leaves much to be desired. Shot blasting work hardens the surface of such forms which adversely affects their cold-workability. Also, shot blasting leaves a metallic residue on the surface which can cause corrosion unless it is removed by acid cleaning. The drawability and extrudability of an elongated steel alloy form are adversely affected when the surface is excessively cratered and torn by the shot blasting process. Furthermore, the surface indentations provided by shot blasting are relatively sharp, jagged voids that can easily trap lubricant, precoat, or coolant used during the cold working process. During elevated temperature thermal treatment of the elongated form after cold working, localized corrosive attack can occur in the indentations, thereby resulting in undesirable surface pitting.
Apparatus for shot blasting is relatively massive and is not conducive to in-line or single strand processing. Also, a significant amount of energy is required to operate a shot blasting apparatus because shot blasting is an airless process that uses a large centrifugal wheel to propel the metal shot.
Another known method of enhancing the cold-workability of an elongated steel alloy form is to coat it with a thin layer of a more malleable material such as copper. Although copper coating can significantly enhance the cold-workability of many steel alloys, the coating must ultimately be removed from the cold-worked product. The removal of the copper coating is costly and poses significant problems in connection with environmental safety because it requires the use of highly corrosive chemicals.
Air-driven abrasive media such as sand, aluminum oxide, or glass beads have been used for treating the surface of a metal article to remove oxide scale, paint, or dry surface residues and to condition the metal surface for good adhesion of a paint or coating. In particular, air-driven glass beads have been used for peening metal surfaces to provide a beneficial compressive stress in the peened surface layer. Air-driven glass beads have also been used to provide a dull or matte finish on steel alloy strip that was flat-rolled from round wire.